A method is known (see Ref.1--USSR Author's Certificate No. 179,324, Cl. C09d) for preparing derivatives of pyrazolinylnaphthalic acid having the general formula ##STR3##
where
Compounds of this general formula are organic luminophors with emission in the orange-red and red range of the visible spectrum.
The above-mentioned compounds are prepared proceeding from condensation products of 4-acetylnaphthalic anhydride with unsubstituted or substituted o-phenylene diamine in glacial acetic acid. The yellowish green crystals formed are filtered off and recrystallized from acetic acid. The acetyl derivative of 1,8-naphthylene-1',2'-benzimidazole obtained is treated with aromatic or heterocyclic aldehyde in ethanol in the presence of sodium hydroxide, stirred at ambient temperature, and diluted with water. The precipitate formed is separated by filtration and recrystallized from acetic acid. An unsaturated ketone is obtained. The ketone is treated with phenyl hydrazine or its substitute in glacial acetic acid under reflux, the crystals precipitated from the cooled solution are filtered off, washed with methanol, and dried. The product is obtained as water-insoluble red or dark-red crystals soluble in conventional organic solvents. The ready (commercial grade) product yield is from 62 to 85%. Luminescence characteristics: in toluene, .lambda..sub.max =598 to 610 nm; in crystal form, .lambda..sub.max =620 to 670 nm.
Another method has been also described (see Ref.2--USSR Author's Certificate No. 196,873, Cl. C09d) for preparing derivatives of pyrazolinylnaphthalic acid having general formula ##STR4##
where
The method consists of a first synthesis stage, in which a mixture of 4-acetylnaphthalic anhydride, aniline, and glacial acetic acid is heated under reflux, the reaction mixture is cooled, the precipitated gray crystals are separated by filtration, treated with hydrochloric acid solution, then with hot sodium carbonate solution, and recrystallized from acetic acid. Acetyl derivative, the 4-acetylnaphthalic acid phenylimide is obtained.
At the second stage, the acetyl derivative, the 4-acetylnaphthalic acid phenylimide, is converted into an unsaturated ketone. To that end, 4-acetylnaphthalic acid phenylimide, ethanol, sodium hydroxide solution, and benzaldehyde are stirred at ambient temperature. The precipitate formed is separated by filtration, water washed till neutral reaction (litmus), dried, and re-crystallized from toluene. The product is obtained as yellow crystals.
At the third synthesis stage, the unsaturated ketone is treated with phenyl hydrazine in a mixture of ethanol and sodium hydroxide solution under reflux. The crystals precipitated from the mixture under cooling are separated by filtration, water washed till neutral reaction, and treated with boiling alcohol. The product obtained is purified using chromatography of toluene solution. The final product is obtained as red crystals at yield of 57 to 64%. The crystals are water-insoluble and soluble in organic solvents. Luminescence: in toluene, .lambda..sub.max =570 nm.
The methods for preparing luminophors disclosed in the above-mentioned Author's Certificates are almost identical with each other except for that 4-acetylnaphthalic anhydride is treated with aniline to obtain orange-red luminophors (Ref.2) while to produce red ones, the same anhydride is reacted with o-phenylene diamine.
In this reaction, the same drawbacks are typical of both methods, namely:
(A) Low quality of the final product. The product contains considerable amounts of impurities, mainly unsaturated naphthalic acid derivatives that cause green fluorescence. The presence of such impurities in the final product is due to the preparation method itself. The final product purification from the impurities mentioned is associated with difficulties in the process, increased production costs, and substantial losses of the final product. To obtain high-quality luminophors, the chromatographic purification is required resulting in a loss of 30 to 40% of the final product. In Ref.2 the yields of various red range luminophors are said to be from 62 to 85%. The real useful product yield after the purification is, however, only from 35 to 50%.
(B) The process is difficult to perform and laborious. When producing orange, orange-red, and red luminophors, the corresponding intermediates, i.e., acetyl derivatives, are to be prepared every time. For example, 4-acetylnaphthalic anhydride is to be condensed with aniline preparing 4-acetylnaphthalic acid phenylimide to produce luminophors emitting in orange range, while to obtain red luminophors, the same anhydride must be condensed with o-phenylene diamine preparing 4(5)-acetyl-1,8-naphthoylene-1',2'-benzimidazole. These preparations make the process difficult and increase its cost.
(C) Moreover, the process is complicated by the purification of intermediates both at the stages of their synthesis and at the preparation of final products.
Another method is known for preparing luminophors of the structure and emission region similar to the subject of this application; the method consists in that 4-cinnamoyl-1,8-naphthylene-1',2'-benzimidazole is condensed with o-tolyl hydrazine in acetic acid medium under boiling, i.e., similar to the methods described above. The precipitate formed is separated by filtration, dried, and purified by the column chromatography. By this method, 4-[1-(2-methylphenyl)-5-phenyl-2-pyrazolinyl-3]-1,8-naphthoylene-1',2'-ben zimidazole with the structure shown is obtained. ##STR5##
The luminophore compounds so prepared are fluorescent in the orange-red spectral region with a large Stokes shift: in toluene, absorption at .lambda..sub.max =470 nm, luminescence at .lambda..sub.max =595 nm. (See Ref.3--USSR Author's Certificate No. 1,148,291, Cl. C09K 11/06).
The use of the method of the present invention will permit the synthesis of the named luminophores to be carried out faster with a higher yield of the product. That will eliminate the extra cost related to the use of chemicals and labor and will provide for the more economical synthesis of the product.